In general, a pneumatic tire is provided with blocks arranged on its tread surface in the circumferential direction at pitches with different pitch lengths in order to reduce various noises produced by patterns of the tire. The blocks disperse pitch noise in such a wide frequency that the noise performance of the tire is improved. However, when the blocks are arranged in the tire circumferential direction at different pitches as described above, there occurs a difference in rigidity between a block on a pitch with a large pitch length and a block on a pitch with a small pitch length, which produces a problem that the difference in rigidity causes vibration in the tire.
To be more precise, for a tire in which blocks are arranged in the tire circumferential direction at pitches having different pitch lengths, the pitches are usually set to have the same groove area ratio per pitch. Therefore, the larger the length of a pitch is, the larger the width of a lateral groove is. Accordingly, as shown in FIG. 5, when a tire is manufactured, a mold M largely pushes a tread rubber G downwardly below the grooves on the pitches with a large pitch length. Thus, the larger the length of a pitch is, the larger the gauge thickness below the groove is. This is the cause for making a pitch length difference vary the rigidity between blocks.
To eliminate the above problem, it has been proposed that the varying of the gauge thickness of the rubber under the grooves due to the difference between pitch lengths is suppressed in a way that a lateral groove on a pitch with a larger pitch length is set to have a larger groove wall angle which is formed by the groove wall and a direction normal to the tread surface (e.g., refer to Patent Document 1). Further, it has been proposed that the varying of the gauge thickness of the rubber under the groove due to the difference between pitch lengths is suppressed by setting a pitch with a larger pitch length to have a smaller groove area ratio (e.g., refer to Patent Document 2). According to either of these proposals, however, the tread pattern rigidity is uniformed over the entire circumference of the tire only to a limited extent.    [Patent Document 1] Japanese Patent No. 3618767    [Patent Document 2] Japanese patent application Kokai publication No. 2004-210133